Strictly Speaking Feb 14 is the Catholic Church Feast day of St Valentine of Rome who was put in prison and died there for performing marrage rituals for solders in the Roman Legons (Against the order of the emperor who felt single men fought better).

It was promoted by the Catholic Church as a clean wholesome subsitute for the Veneralia, a Feast to Venus, which got quite kinky, Or was it the Roman fertility goddess Juno's Feast day, I beleive it is a debated topic.